1. Field of the Invention
The present invention relates generally to the field of semiconductor fabrication and, more particularly, to the fabrication of a multiple-gate transistor.
2. Description of Related Art
A conventional metal-oxide-semiconductor (MOS) transistor is typically characterized by a structure in which a gate electrode is displaced above the transistor channel region by an intermediate gate dielectric film. The region below the channel may include the bulk substrate or an epitaxial film. The transistor is operated by applying a bias to the gate electrode. The bulk material is likely grounded or biased to a constant voltage. Thus, the conventional transistor may be described as having a single-sided gate since the gate exists on only one side of the channel.
It is generally recognized that single-sided gate transistors inherently exhibit operational characteristics, including leakage current, drive current, and sub-threshold slope, that are less than ideal. These parameters are particularly critical in low power applications such as wireless technology. Multiple-gate transistor structures, in which gate dielectrics and gate electrodes are formed on two (or more) sides of the transistor channel, have been proposed to address this problem.
Typical multiple-gate transistor structures employ a xe2x80x9csymmetricxe2x80x9d design in which the gate electrode on both sides of the channel is the same material, doping, conductivity, and so forth. Similarly, the gate dielectrics on both sides of the channel are the same. Symmetric multiple gate transistors have been found to suffer from a threshold voltage control problem. For example, n-channel threshold voltages in symmetric multi-gate sub-micron transistors using p+ poly gates and undoped or lightly doped channels have been found to be in the range of 0.9 V whereas threshold voltages of approximately 0.1 to 0.4 V are required for optimum performance. Accordingly, it would be desirable to implement a process enabling the formation of asymmetric, multiple-gate transistors. It would be further desirable if the implemented process did not substantially add to the complexity of the process and did not require the use of new materials and/or equipment.